I would just add that dacron soaks up water so if using in the sea, it can hold salt water against the spool. On a good quality reel its probably not much of a problem, but I have had a cheaper reel corrode under the backing.
I have very heavy (150lb) gel spun on my salt water reel only because I do not need several hundred yards of backing. 150 is still thinner than 30lb dacron, but I only need around 100 yards, more than enough for the uk.